Magnetic Resonance Imaging (MRI) is an imaging technique based in part on the absorption and emission of energy in the radio frequency range. To obtain the necessary magnetic resonance images, a patient (or other target) is placed in a magnetic resonance scanner. The scanner provides a magnetic field that causes magnetic moments in the patient or target atoms to align with the magnetic field. The scanner also includes coils that apply a transverse magnetic field. RF pulses are emitted by the coils, causing the target atoms to absorb energy. In response to the RF pulses, photons are emitted by the target atoms and detected as signals in receiver coils.
In several forms of MRI, it is desirable to obtain a time series of images. For example, diffusion weighted imaging (DWI) offers a means to evaluate an area of anatomy in terms of the motion of water molecules. The level of water diffusivity can provide an indication of the structure of the tissue at the cellular level. In terms of tumors within an organ such as, for example, the liver, the water diffusivity within the tumor is less than that of the healthy tissue of the organ because the cancerous cells are denser with more impediments to water motion. In a necrotic tumor, which has undergone treatment to kill cancerous cells, there is an increase in diffusivity compared to a viable tumor because in dead cancerous cells the membranes are broken down allowing greater motion of water molecules. Thus, measurement of water diffusivity can serve as a surrogate marker to evaluate tumor treatment response.
To obtain a proper dataset for DWI, a number of images are captured over a predetermined period of time (e.g., 10 images in 20 seconds). When multiple image slices are acquired for each dataset, it is desirable that the complete dataset be captured during one breath-hold to avoid motion of the anatomy. However, if the scan duration is too long, then these scans are generally done under free-breathing conditions with image registration being performed in post-processing. Physiological motion, such as motion from breathing, heart motion, and other tissue motion, cause intensity reductions in the resultant images and overwhelm water diffusivity measurements.
In order to reduce such intensity loss, Temporal Maximum Intensity Projection (TMIP) can be used, which is an extension into a temporal domain of a widely accepted volume rendering technique (MIP) that extracts high-intensity structure from volumetric scalar data. In TMIP, the highest sample value encountered across images is determined at each pixel over the time series. MIP approaches are commonly used to extract vascular structure from medical CT or MRI data sets and exploits the fact that the data values of vascular structures are higher than the values of the surrounding tissue. By depicting the maximum data value seen through each pixel, the structure of the vessels contained in the data is captured.
Even using TMIP, the resultant image can contain substantial noise. In particular applications of MRI, such as DWI, TMIP has been found to be especially ineffective.